This invention relates to electro-mechanical measuring apparatus for use with high field strength superconducting magnets and in particular to a probe for measuring relative displacement between the magnet coils and the outer mounting surfaces of the cryostat of a high field strength superconducting magnet.
Superconducting magnets find commercial use in magnetic resonance imaging (MRI). In MRI, the protons of an imaged body are excited into resonance by a radio frequency field applied to the imaged object in the presence of a static magnetic field. The static magnetic field may be produced by a superconducting magnet having multiple coils of superconducting wire immersed in a cryogen and energized with an electrical current. Field strengths of several Tesla may be achieved with essentially no power consumption.
The frequency of the resonance of the protons of the imaged object excited by the radio frequency field is dependant on the strength of the magnetic field and certain characteristics of the protons.
As the protons precess in resonance, separate gradient magnetic fields of substantially smaller strength than the static magnetic field are applied to the imaged body to shift the phase and frequency of the resonance of the protons in accordance with each proton's location within the imaged object. The combined signal produced by the resonating protons is then analyzed mathematically to produce an image of the imaged body along a "slice" through the imaged body.
The contribution of each resonating proton to the slice image is dependant of the phase and frequency of its resonance. If the static magnetic field is uniform, this phase and frequency will be dependent solely on the position of the protons in the gradient magnetic field. If the static magnetic field is not uniform, the apparent position of the protons, as determined by the phase and frequency of their resonance, will be shifted. This introduces artifacts or other distortions into the reconstructed image of the imaged body. The elimination of such artifacts requires that the static magnetic field used in MRI must be extremely uniform. Magnetic field homogeneities of less than a few parts per million over the imaging volume are required.
It follows that the static magnetic field also must be highly stabile. The time required to collect the data for a single MRI slice image may be several minutes for certain imaging techniques. Accordingly, fluctuations of the static magnetic field in time will also introduce artifacts and distortions to the image.
One cause of instabilities in the static magnetic field is mechanical motion of the superconducting magnet coils within the magnet support structure or cryostat.
Motion of the magnet coils is a particular problem for MRI equipment used in mobile applications. This MRI equipment is attached to a mobile trailer rather than to the foundation of a building and hence is subjected to considerably more environmental vibration.
The coils of the superconducting magnet are immersed in a cryogen, typically liquid helium and held fixed within a helium vessel. The cryogen is at temperatures near 4.degree. K. and must be insulated from the ambient temperatures of approximately 300.degree. K. This is accomplished by a vacuum shell and a series of heat shields forming a cryostat that surrounds the helium vessel to reduce heat flow inward to the helium vessel.
The helium vessel may be suspended within the cryostat by a series of tensioned supports attached to the outer shell of the cryostat. These supports provide a path for heat flow between the outer wall of the cryostat and the helium vessel and hence are insulated and minimized in number. Generally, in determining the number and size of the supports, there is a tradeoff between designing the support system to hold the helium vessel rigidly with respect to the outer shell of the cryostat and designing the support system to reduce the amount of heat leakage along the supports to the helium vessel. In mobile applications, there is a tradeoff between designing the support system to hold the helium vessel rigidly with respect to the outer shell of the cryostat and designing the support system to resist the shock of travel. The ability to accurately measure the vibrational susceptibility of a magnet coil support system would be valuable in making these tradeoffs.
Measuring a high field superconducting magnet's resistance to vibration is difficult. As mentioned, the helium vessel is surrounding by radiation shields and held within a vacuum at superconducting temperatures. For a 1.5 Tesla magnet, the magnetic field strength within the magnet may be as high as 4.5 Tesla or over 500,000 times the strength of the earth's magnetic field. In short, the helium vessel is not readily accessible to conventional measuring instruments.